Multi-component said mass exchange as presently practiced entails numerous problems and difficulties, as indicated below.
The exchange of mass between liquid and vapor can be conducted cocurrently, countercurrently, or crosscurrently, analogous to heat exchange processes. Countercurrent mass exchange can achieve complete separations and correspondingly maximum temperature glide (the change in equilibrium temperature associated with a change in concentration). Cocurrent mass exchange is severely limited both as to the degree of separation achievable and the temperature glide resulting from absorption. By increasing the number of crosscurrent stages, crosscurrent mass exchange can be made to nearly approach countercurrent performance.
The exothermic multi-component fluid mass exchanging processes, i.e. those in which there is a net transfer of mass from vapor to liquid phase, exhibit high resistance to mass transfer owing to a concentration gradient in the liquid film. When the vapor is also multi-component, a similar concentration gradient exists in the vapor phase, which in many cases is an even greater resistance to mass transfer. These effects are also present in the endothermic processes, e.g. desorption or stripping. When the endothermicity (i.e. the supplied heat) causes nucleate or bulk boiling at the heated surface, the difference in concentration between the vapor and liquid phase causes large concentration gradients locally. However this effect is offset by the mixing effect that the moving bubbles have on the liquid film.
Fully countercurrent processes such as conventional fractional distillation have the problem that most of the vapor must pass through most of the countercurrent stages, even though a very small fraction of vapor is actually exchanged within each stage (or transfer unit). Thus very large column diameters are necessary, with loadings constrained by flooding velocity or entrainment. The conventional countercurrent configuration uses gravity downflow of liquid, and hence the stages (or packing) are stacked vertically, frequently extending 30 meters or more in height. Diabatic distillation is much more energy efficient than adiabatic distillation, since the input heat can be on average colder and the reject heat on average warmer (less entropy generated). Presumably the reason diabatic fractional distillation is extremely rare are: the added resistance to mass exchange it entails, coupled with the need to disperse a heat exchanger over the extended height of typical columns, plus little change in first law efficiency. Thus the criterion of minimization of capital cost leads to quite energy inefficient distillation as currently practiced.
The benefits supplied by liquid recirculation to single component heated mass exchange processes, e.g. steam boiling, are widely known and practiced. The liquid recirculation causes higher liquid to vapor ratios in the boiling channels, enabling cocurrent upflow boiling while keeping the walls wetted at higher velocities, thereby causing thinner liquid films, more mixing, and more uniform temperatures. Liquid recirculation has also been applied to boiling a two component mixture (T. Fukuchi, "Development of Small-Sized Double Effect Gas Absorption Chiller/Heater," Proceedings 19th International Congress of Refrigeration Volume III b, p. 782-789, August 1995, and U.S. Pat. No. 4,127,993). In the U.S. Pat. No. 4,127,993 disclosure one downcomer serves one group of upflow channels, and a second one serves a second group. The two groups are not pressure equalized via a common vapor space. Additional examples of cocurrent upflow in the channels of an absorption cycle generator can be found in U.S. Pat. No. 5,435,154 and elsewhere. Cocurrent upflow is also known in absorption cycle absorbers, e.g. U.S. Pat. No. 5,339,654.
A multiple stage diabatic absorber with crosscurrent feed of vapor to each stage (called an "adiabatic regenerator") is disclosed in U.S. Pat. Nos. 4,921,515 and 5,157,942. Four absorption stages are illustrated, alternating between cocurrent upflow and countercurrent. The stages are not pressure equalized, hence the pressure increases each stage owing to forced serpentine circulation of liquid through the stages. Since the vapor is supplied crosscurrently, all of the vapor must be above the pressure of the highest pressure stage.
Absorption cycle processes and apparatus such as space conditioning systems, refrigeration systems, and heat pumps present a special case of multi-component fluid mass exchange requirements. A circulating liquid sorbent exothermically absorbs vapor and subsequently endothermically desorbs vapor. When the sorbent is volatile, rectification and/or stripping may also be called for. In many cases a large temperature glide is desirable with desorption--to extract more heat from a heat source, to reduce corrosive conditions, to reduce subsequent rectification requirements, and other reasons. Similarly it is frequently desired to achieve a large temperature glide in absorption--to create larger driving forces for heat rejection, to enable higher temperature recovery of useful heat, and other reasons, such as achieving temperature overlap between absorption and desorption. A cycle wherein heat is internally transferred within that overlap is termed a GAX cycle. That transfer of heat has proven to be a difficult problem for volatile sorbents with countercurrent heat and mass transfer, owing to the "gravity mismatch": the desorber with falling liquid sorbent is hottest at the bottom, whereas the absorber with falling liquid sorbent is hottest at the top. Countercurrent operation in the absorber (and hence falling liquid) is desirable because that yields the greatest temperature glide, thus resulting in greatest temperature overlap. Larger temperature overlap makes possible more internal transfer of heat, thus increasing cycle Coefficient of Performance (COP). Countercurrent operation in the desorber is desirable as it reduces or eliminates subsequent rectification needs.
The original approach to accomplishing the GAX heat exchange was to use an intermediary hermetic heat transfer loop. Although this retained the desirable countercurrent configuration of both sorptions, it imposed an additional heat transfer temperature differential, thus negating much of the objective of maximizing the temperature overlap. Also an additional fluid loop with precise and variable flow was required. Hence over the past decade the norm has shifted to a direct exchange of GAX heat between sorption cycle fluids.
One early disclosure changed both GAX sorptions to fully cocurrent--(U.S. Pat. No. 4,311,019), thus falling liquid and the attendant vertical configuration was no longer required. Cocurrency yields good transfer coefficients, but severely limits the temperature glide of absorption, and hence reduces the temperature overlap and COP. More typical has been to conduct only the GAX desorption cocurrently, while retaining countercurrent (falling liquid) GAX absorption. This preserves the full GAX temperature overlap, but requires much greater rectification following the GAX desorption. Most cycle disclosures of the past decade have been this type, including U.S. Pat. Nos. 4,846,240 and 5,367,884. Unfortunately this approach retains the difficult mass transfer associated with countercurrent absorption when used with a volatile sorbent. Conversely, combining cocurrent absorption with countercurrent desorption yields significant hardware advantages but imposes a major thermodynamic disadvantage (owing to the concentration mixing).
Thus it would be highly advantageous, and included among the objects of this invention, to achieve both absorption and desorption in an absorption cycle apparatus in such a manner that the thermodynamic benefit of countercurrent operation is nearly achieved without the normally accompanying mass transfer difficulties. It is further desired this be accomplished with simple, low-cost enhanced heat and mass transfer apparatus. It is most desired that countercurrent heat exchange be directly possible in a GAX component comprised of such an absorber and desorber. Examples of low-cost enhanced transfer surface are brazed plate fin; stamped fins pressed on cylinders, and plate fin fitted within an annular space between pressure vessels.
Absorption cycles generally and particularly GAX cycles have very low liquid flow rates relative to the vapor flow rate, and as a result require exceptionally good liquid distributors. Furthermore, the achievement of the desired temperature glide in a vertical direction (countercurrent operation) requires more height than is normally allowable for small scale space conditioning apparatus. Another common problem in absorption cycle absorbers is excessive subcooling of the liquid sorbent. Desirably all three of these problems are also solved by the invention.
It would be desirable, and included among the objects of this invention, to achieve the high exchange efficiency benefit of cocurrent upflow sorption without at the same time suffering the various disadvantages of prior art disclosed approaches:
Also included among the objects of this invention are achievement of a near approach to the thermodynamic benefit and large temperature glide obtainable with countercurrent vapor-liquid contact, without the attendant prior art disadvantages of:
The energy benefits of conducting fractional distillation stripping and rectification diabatically, plus also particular hardware configurations which make it possible to do so, have been disclosed in U.S. Pat. Nos. 4,025,398, 3,756,035, and 3,508,412.
The advantages of locally cocurrent, globally countercurrent distillation, plus also one particular enabling hardware configuration, are disclosed in U.S. Pat. No. 4,361,469. In particular, that invention achieves intensification and compactness with regard to column diameter, owing to avoiding the flooding limitation in part of the apparatus. However there is no reduction in the required number of vertical trays, and the tray height may even be higher owing to the need for a vapor-liquid separator above each tray. Furthermore the tray pressure drops are cumulative, i.e. the trays are not pressure equalized via a common vapor space, and also most of the vapor must traverse most of the trays. Thus it would also be advantageous and an object of this invention to accomplish fractional distillation, stripping, rectification, and/or reverse distillation diabatically in more compact, lower height and lower cost apparatus.